Plasma lipoproteins and Coronary artery disease. Epidemiological studies indicate that increased plasma cholesterol levels increase the risk for atherosclerosis. Five completed major trials have provided conclusive evidence of a benefit from treatment aimed primarily at reducing low-density lipoprotein (LDL)-cholesterol (Illingworth R. D., et al. Current Opini. Lipidol. 1999, 10:383-386). Among other lipoprotein risk factors is familial dysbetalipoproteinemia, which results in the accumulation of remnant atherogenic lipoproteins derived from the catabolism of chylomicron and VLDL (Kwiterovich, P. O., Jr. Am. J. Cardiol. 1998, 82:3U-7U). It has been shown that a 1% decrease in the plasma cholesterol level decreases the risk of coronary artery disease by 2% (Deedwania, P. C. Med. Clin. North Am. 1995, 79:973-998). The focus of angiographic trials has been on LDL reduction and these studies have demonstrated that decreases of LDL-cholesterol of more than 30% to 35% are associated with lower rates of coronary events (Watts, G. W., et al. Atherosclerosis 1998, 414:17-30). There is also growing evidence that triglyceride-rich lipoproteins may adversely affect endothelial function and increase oxidative stress by promoting the production of small, dense LDL and by reducing high-density lipoprotein (HDL) levels (Marais, D., Curr. Opin. Lipidol. 2000, 11:597-602).
Anti-atherogenic properties of Apolipoprotein E (Apo E). Apolipoprotein E is a protein that binds lipid and has two major domains (Mahley, R. W., et al. J. Lipid Res. 1999, 40:622-630). The 22 kDa amino terminal domain has been shown by X-ray crystallographic studies to be a 4-helix bundle (Wilson, C., et al. Science 1991; 252:1817-1822) and to contain a positively-charged receptor binding domain. For this region to mediate very low-density lipoprotein (VLDL) binding to its receptors, the apolipoprotein must associate with the lipoprotein surface; this is enabled by the C-terminal amphipathic helical region. If the 4-helix bundle that contains the positively charged receptor-binding domain does not open up on the lipoprotein surface, then the VLDL is defective in binding to receptors. Thus, the positively charged arginine (Arg)-rich cluster domain of the Apo E and the C-terminal amphipathic helical domain, are both required for the enhanced uptake of atherogenic Apo E-containing lipoproteins.
Chylomicron is a lipoprotein found in blood plasma, which carries lipids from the intestines into other body tissues and is made up of a drop of triacylglycerols surrounded by a protein-phospholipid coating. Chylomicron remnants are taken up by the liver (Havel, R. J., 1985, Arteriosclerosis. 5:569-580) after sequestration in the space of Disse, which is enriched with Apo E (Kwiterovich, P. O., Jr., 1998; Deedwania, P. C., 1995; and Watts, G. W., et al., 1998). Apo E is the major mediator of hepatic remnant lipoprotein uptake by the LDL receptor or LRP. Lipolysis of normal VLDL Sf (subfraction) of more than 60 permit binding of the lipolytic remnant to the LDL receptor (Catapano, A. L. et al. 1979, J. Biol. Chem. 254:1007-1009; Schonfield, G., et al. 1979. J. Clin. Invest. 64:1288-1297). Lipoprotein lipase (LpL) may facilitate uptake through localization of Apo B-containing lipoproteins to membrane heparan sulphate proteoglycan (HSPG)(Eisenberg, et al. 1992. J. Clin. Invest. 90:2013-2021; Hussain, M., et al., J. Biol. Chem. 2000, 275:29324-29330) and/or through binding to the LDL-receptor-related protein (LRP)(Beisiegel, U., et al., 1989, Nature 341:162-164). Cell-surface HSPG may also function as a receptor and has variable binding affinities for specific isoforms of Apo E. In particular, Apo E is synthesized by the liver and also by monocyte/macrophages, where it exerts its effect on cholesterol homeostasis. In vivo evidence for the local effect of lack of Apo E comes from the observations of Linton and Fazio, who showed accelerated atherosclerosis in C57BL/6 mice transplanted with bone marrow from Apo E-deficient mice (Linton, M. F. and Fazio, S. Curr. Openi. Lipidol. 1999, 10:97-105). Apo E-dependent LDL cholesteryl ester uptake pathway has been demonstrated in murine adrenocortical cells (Swarnakar, S., et al. J. Biol. Chem. 2001, 276:21121-21126). This appears to involve chondroitin sulphate proteoglycan (CSPG) and a 2-macroglobulin receptor.
It has been shown that the receptor-binding domain of Apo E, rich in Arg residues (141-150), covalently linked to a synthetic class A amphipathic-helical domain, enhances the hepatic atherogenic lipoprotein uptake (Datta, G. et al. Biochemistry 2000, 30:213-220). Recent studies indicate that a potential anti-atherogenic action of Apo E is that it stimulates endothelial production of heparan sulfate (HS)(Paka, L., et al. J. Biol. Chem. 1999, 274:4816-4823). Lipoproteins are complexes of one or more lipids bound to one or more proteins and transport water-insoluble fats in the blood. Cholesterol is carried through the bloodstream by lipoproteins. There are no agents available which reduce cholesterol via the binding mechanisms of lipoproteins. There is a need for more effective agents that are capable of reducing cholesterol in a subject so as to reduce diseases and conditions which are associated with increased cholesterol.